Intra-operative ex vivo photoacoustic nodal staging in a rat model using a clinical superparamagnetic iron oxide nanoparticle dispersion.

The ability to accurately detect tumor metastases in lymph nodes is essential for intra-operative staging of various malignancies. Histopathological assessment of nodes has the drawback of a time delay before results are available to the surgeon and a likelihood of missing metastases. Photoacoustic (PA) imaging has been shown to possess the potential to detect melanoma metastases in resected in toto lymph nodes based on intrinsic contrast. To extend application of the method to other malignancies, extrinsic contrast for lymphatic mapping is important. We investigate in a metastatic animal model whether clinically approved superparamagnetic iron oxide (SPIO) nanoparticles, applied for MRI, can help PA imaging for staging in an intra-operative ex vivo setting. Imaging results are compared with 14 Tesla MR images and histology. We observe that irregularities in SPIO distribution in PA images of the nodes and a decrease in contrast correlate with metastatic involvement as seen in MR images and histology. The results show that a PA based imaging technique may be valuable for nodal staging in the field of surgical oncology.